Some circuits require low-voltage and high-voltage devices on the same chip. Examples include EPROM and EEPROM devices, which require high-voltage transistors for programming, and low-voltage logic devices which require high-voltage transistors to handle higher external power supply voltages. Due to the different voltage requirements for these two types of devices, the gate oxides need to have different thicknesses. More specifically, the high voltage transistor requires a thicker gate oxide to support the higher voltage, whereas the low-voltage transistor requires a thinner gate oxide. The standard process for producing different gate oxide thicknesses is to pattern the high voltage gate oxide and to remove this oxide from the low voltage areas by etching. The process of patterning directly on the gate oxide causes a reduction in yield.
It is the object of the present invention to provide sub-micron high-voltage and low-voltage transistors on the same chip without having to pattern and etch the high voltage gate oxide.